Large fabric bags are commonly used for transporting bulk quantities of particulate material. Each bag is large and heavy enough to require lifting by mechanical equipment having hooks or tines. One example of such equipment is a forklift truck.
These advantages of large fabric bags for the carrying particulate material are clear. These bags are easily filled, stored, and transported. After the particulate material has been emptied from the bags, the bag can be folded for ease of storage and reuse. It is very critical that each bag be sufficiently strong to hold a substantial amount of material. It is common for such bags to hold in excess of 2,000 kilograms each.
The named inventor of this application is the inventor or coinventor of a number of other patents in this field. His experience in this field causes his company to be a highly recognized leader in the field of bulk material handling. These patents include, but are not limited to U.S. Pat. Nos. 5,738,619 and 4,646,357.
Typically, the bags are desired to be of uniform square, rectangular, or cylindrical horizontal cross-section. The circular cross-section is more efficient use of material, relative to volume of the container and the surface area of material used to form the container as a cylinder. Transport efficiency is more effective with the square cross section. This square efficiency is related to the shape of the transport or shipping containers, which transport containers are not cylindrical.
Generally, bags are required to be made of expensive, heavy-duty material to achieve desired strength for carrying the weight of material. The fabric used for these bags is required to have a weight of at least five ounces per square yard (150 grams per square meter) and a tensile strength of at least 220 pounds per square inch (28.2 kilograms per square centimeter). Such a strong fabric is required to make a bag of sufficient strength.
As of now, a bulk lift bag, formed from a woven fabric, requires that the fabric be coated, so the bag might hold fine particles. Coating is expensive, and often peels off and gets mixed with the product contained in the bag by the end users, especially a particulate product being removed from the resulting bag. Additionally the sewing, required to form the fabric into the bag, produces apertures in the fabric, which, in turn permits the powder or fine particle to flow through the apertures and out of the bag.
While the coated bag requires no liner, the sewing apertures and the stretching of the bag upon filling, storing, and transporting, cause sifting problems. The sewing process causes apertures in the lining and allows the powder to either cake, or flow out of the bag, through the sewing apertures.
Sometimes, a material, suitable for use as seat belts or other narrow strips of fabric, is placed in between the sewn sections of the bulk bag and secured therein. This structure, designed to avoid leaking of fine powders from the bulk bag, does not work, because the hole elongates when a filled bag is lifted. Fine powders can just pour out through the elongated holes.
Still a further problem, with addition of this strip of material to the seams, occurs as the manufacturing process slows to one-half of manufacturing speed. In other words, the addition of this thin strip of to the manufacturing process cuts production of bags per hour by about one-half. As a result, production costs greatly increase and create a problem.
There are many other problems with making sift proof bags to contain very fine particles of material. Typical bulk lift bags are usually woven from polypropylene. It is known to put a polyethylene bag formed from a film inside a polypropylene bag in order to keep the powder or other contents dry and contained within the bulk bag, but these methods are clumsy expensive, wasteful and unsuccessful.
This liner of the prior art must have a substantial, excess amount of material, in order to avoid the stretching and forcing the liner to tear, while the lined bag is being filled and transported. Because of this excess material, loading and unloading of the bulk lift bag can be made more complicated.
After the bag is filled, the excessively sized liner, which is about 150 inches tall (380 centimeters) in circumference, for the bag extends well over the top of the bag. The excess top portion is then tied. When emptying the bag, this liner causes a problem. As the bag is opened at the bottom, the liner slides down and clogs up the auger as it dispenses the powdered material from the bag.
The liner material can be torn and can get mixed with the material or particles being removed from the bag in the auger, resulting in damage, both to the auger and to the material carried in the bag. This is especially important when food or a specialty material is carried in the bag.
Clearly, the bulk lift bag is highly advantageous as a transport mode. The sifting problem is a major difficulty in transportation of fine particles. A simplified correction has yet to be found.